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Top2. Topology Configuration
The Horizontal Axis Wind Turbine (HAWT) configuration, where the direction of the airstream is parallel to the rotation of axis and the main driving force is lift, is found to be considerably more suitable mainly due to cost and efficiency advantages. The rotor will be facing the wind in the upwind position, with 3 blades mounted on a rigid hub, resulting in a simple and cost effective design. Regarding the rotor speed, variable speed wind rotors are a norm in modern small wind applications, rather than exception. They increase the cost of the project but they increase its efficiency as well and therefore this is the chosen option in this case (Vlasakoudi, 2012).
Additional concerns of the topology configuration of a small wind rotor include the material of the blades, the power control, the implementation of gearbox and the generator type of the turbine.
According to manufacturers and suppliers of small wind rotor blades, glass fibre composites (GFC) is a durable reinforced fiberglass material with a high power coefficient, Cp=0.49 (KM, 2009; Euros, 2012). In terms of power control, the pitch control concept is chosen to limit the power output by putting a ceiling on the rotor speed and thus output power as the wind velocity increases as well as the yaw control concept, to optimise power by rotating the entire wind turbine to face the oncoming wind. Another important factor to be determined is the optimal tip speed ratio of the rotor λopt designed. The formulas to calculate its value incorporate experimental data, which are beyond the scope of this project. Therefore, the λopt is chosen to be equal to 7, with respect to tables and plots available in literature (Vlasakoudi, 2012).
Once the optimum tip speed ratio is determined, the rotor can function near peak efficiency at all viable wind speeds (Vlasakoudi, 2012). Finally, the most favorable gearbox and generator options for a small wind rotor, is a gearless machine that drives a permanent magnet generator PMG. The choice of PMG is also beneficial for variable speed rotors and exhibits high efficiency levels, up to 93% (Bumby et al., 2006).
The tilt-up configuration is considerably more suitable for the purpose of this project. This is consistent with the general sentiment of the industry and texts. The rigid hub is chosen due to its simplicity. Both teetering and hinged hubs require complex bearing arrangements which have a negative impact on maintenance, reliability and project cost. Rigid hubs are the most common configuration regarding 3 blade rotors.